no they don't who told you that????? they are only used when your system is asking for more power than your car can produce (dimming lights, etc).....it takes it from the cap instead of your battery since the cap is always charged and ready to deliver the power whenever your subwoofer needs it for example.

I had a digital cap on my system and it worked perfectly i had over a 2 grand custom system in my car and had it for 2 yrs, my battery/alternator never gave me any probs and lights never dimmed even when i maxed it out....(had a 98 2dr ls)